Amidines from thioimidates

Activation of thioamides to thioimidates with alkylation is also applied to the synthesis of amidines. Dijkink et al. [28] applied this method to the synthesis of chiral bicyclic amidines from (5)-malic acid as a key step. However, the amidine was found to be unstable due to isomerization of the imine double bond followed by ehmination of the silyloxy group (Scheme 3.15). 

Scheme 3.15 Synthesis of bicydic amidine from (S)-malic acid via thioimidate...

As already mentioned the action of ammonia, primary and secondary amines on imidoyl halides affords amidinium salts, from which the amidines (3 Scheme 57) can be set free by treatment with bases (see Section 2.7.2.5.3 and 2.7.2.S.4). In imidates and thioimidates the al-koxy groups and alkylmercapto groups respectively are readily replaced by amino groups giving amidines (340). 

Thioimidic esters. These esters are convenient precursors of amidines and they are prepared from nitriles and PhSH in the presence of HBr. 

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